The present invention relates to a sheet metal forming method and a sheet metal forming apparatus for forming a series of bulges, which forms projections and depressions, in sheet metal using a forming apparatus including a die and a punch.
A conventional fuel cell separator is made of sheet metal including a plurality of bulges forming flow paths for hydrogen and oxygen. Japanese Laid-Open Patent Publication No. 2007-48616 and Japanese Laid-Open Patent Publication No. 2014-213343 disclose forming methods and forming apparatuses for such fuel cell separators. The forming apparatuses of these publications each include upper and lower forming dies that move toward and away from each other. The forming dies have forming surfaces including projections and depressions. The projections of one of the forming dies correspond to the depressions of the other die. These depressions and projections form bulges of the sheet metal such that the bulges alternately extend in the opposite directions.
The technique disclosed in Japanese Laid-Open Patent Publication No. 2007-48616 involves a prior stamping process, which forms depressions and projections using upper and lower forming dies so that the sheet metal is corrugated and has a series of curved bulges, and a subsequent coining process, which forms flat surfaces in the top sections of the bulges in the corrugated sheet metal.
The technique disclosed in Japanese Laid-Open Patent Publication No. 2014-213343 involves first to third steps to form sheet metal. The first step performs stretching process to form the top sections of the bulges of the corrugated sheet metal to be thinner than other parts. The second step performs rolling to form the side walls of the bulges. The third step expands the top sections of the bulges.
The technique of Japanese Laid-Open Patent Publication No. 2007-48616 requires the coining process after the stamping process. The technique of Japanese Laid-Open Patent Publication No. 2014-213343 requires three steps of the first to third steps. The techniques of these publications require a plurality of different steps, resulting in lower productivity and extensive production equipment. In addition, a plurality of processing steps is performed on the same sheet metal, possibly inflicting serious damage to the sheet metal. This may reduce the strength and the life of the product made of the sheet metal.
In the technique of Japanese Laid-Open Patent Publication No. 2007-48616, during the prior stamping process, the sheet metal is pressed by the projections of the forming dies but is not in contact with the depressions. Accordingly, a compression force is not applied to the parts of the sheet metal that have been formed, and the projections apply tension to the sheet metal. In the subsequent coining process, since the side walls of the depressions and the projections are raised, the two forming dies moving toward each other do not exert much compression on the side walls of the bulges of the sheet metal, although they exert friction on the side walls.
In the technique of Japanese Laid-Open Patent Publication No. 2014-213343, the first step stretches the sheet metal and forms tall bulges, and these bulges are processed in a different step. That is, the techniques of Japanese Laid-Open Patent Publication No. 2007-48616 and Japanese Laid-Open Patent Publication No. 2014-213343 preferentially apply tension to the sheet metal. This may partially reduce the thickness of the sheet metal and cause necking in the stretched side walls of the bulges or other sections, which degrades the performance of the formed products.
Further, in the techniques of Japanese Laid-Open Patent Publication No. 2007-48616 and Japanese Laid-Open Patent Publication No. 2014-213343, the outer surfaces of the top sections of the bulges of the formed sheet metal are flat. Accordingly, when the sheet metal is used as a fuel cell separator, the area of contact between the outer surfaces and a diffusion layer of the fuel cell may not be large enough. Accordingly, the internal electrical resistance of the fuel cell increases, and the pressure on diffusion layers per unit area increases. Thus, the diffusion layer, which is made of a flexible material, may be excessively deformed. This may degrade the performance of the fuel cell.